1. Fields of the Invention
This invention relates to magnetic actuators, more particularly to magnetic actuators suitable for actuating print wires in wire matrix dot printers.
2. Description of the Prior Art
Conventional magnetic actuators employed in print wire actuators for wire matrix dot printers can be broadly classified into two types. One is a non-polarized type as shown in FIG. 1; and the other a polarized type as shown in FIG. 3.
The former type of magnetic actuator is disclosed, for example, in U.S. Pat. No. 4,240,756, the relevant portion of which is reproduced in FIG. 1. As shown in the figure, the magnetic actuator comprises a generally U-shaped yoke 2 with a field coil 1 surrounding its one leg, and an armature 3 so arranged as to bridge both pole faces 2a, 2b of said yoke 2. The base portion of the armature 3 is pushed against the pole face 2a of said U-shaped yoke 2 by a compressor spring 4 and serves as a fulcrum of the armature 3. The free end portion of the armature 3 is normally biased away from the pole face 2b of the yoke 2 by the action of a wire return spring 5, such that an air gap &lt;A&gt; is formed between the pole face 2b of said U-shaped yoke 2 and the armature 3. Thus, the armature 3 is attracted toward the pole face 2b of the U-shaped yoke 2 when the field coil 1 receives a driving voltage. FIG. 2 shows the relationship between the magnetic flux &lt;F&gt; and the magnetomotive force &lt;NI&gt; generated by the field coil 1 within the range &lt;Fa&gt; of the magnetic flux. When the field coil 1 is de-energized, the armature 3 returns to its initial or reset position by the combined forces of the compressor spring 4 and the wire return spring 5. In the meanwhile, the armature for driving the wire actuator is required to be operated at a higher speed by repeated driving signals applied to the field coil 1 at a cycle of as high as from 0.7 to 1.0 KHz. However, it is known that the larger the mass of the armature, the longer the time required for the armature to respond to the driving signals. Therefore, the armature 3 is required to have a mass as small as possible for effecting high speed operation. For this purpose, an armature of less cross sectional area or thickness is found to be preferable. But unfortunately, the reduction in thickness can adversely reduce the magnetic path for the magnetic flux, rendering the armature liable to become magnetically saturated at only a small magnetic flux. Therefore, sufficient attraction force for wire dot printing is hardly expected with the armature of less thickness, which frequently results in blurred printed dots on a recording medium. Accordingly, in this prior art, the thickness of the armature should be decided rather by the requirement for obtaining sufficient impact power than by the requirement for enhancing the response speed. That is, the thickness of the armature cannot be reduced beyond a certain limit so as to compete with an increasing demand for much higher speed operation of the wire dot printing. On the other hand, the polarized magnetic actuator of the latter type is disclosed, for example, in U.S. Pat. No. 4,351,235. In this type, as schematically reproduced in FIG. 3, a permanent magnet 12 is disposed between parallel yokes 10 and 11 of different lengths. Provided at the free end portion of the longer yoke 10 is a core 13 which extends in parallel with the magnetic polarization direction of said permanent magnet 12 and is surrounded by a field coil 14. The upper end of the core 13 serves as a pole face and is spaced from the free end portion of the shorter yoke 11. An armature 15 secured on the shorter yoke 11 comprises a pole piece 15a and a resilient plate 5b made of elastic and magnetically permeable material. The base portion of the resilient plate 15b is secured to the yoke 11 in a cantilever fashion to be in parallel relation thereto and the free end portion of the resilient plate 15b carries the pole piece 15a which defines on their lower surface a pole face confronting the pole face of said core 13 and which forms on their upper surface means for printing a dot. Under an initial condition when the field coil 14 remains unexcited, the armature 15 is attracted against the restoring force of the spring plate 15b to the core member 13 due to the magnetomotive force of the permanent magnet 12. When the field coil 14 receives a driving voltage for generating a magnetomotive force to cancel that of the permanent magnet 12, said attraction force between the armature 15 and the core member 13 is weakened, which causes the armature 15 to move in the direction &lt;P&gt; due to the restoring force of the spring plate 15b and causes said means on the pole piece 15a to imprint a dot on the recording medium.
In this prior art, however, the electromagnetic force is only utilized to cancel or counteract the magnetic force of the permanent magnet retaining the armature in its reset position such that the dot printing is carried out by the restoring action of the resilient plate 15b and not by the electromagnetic force. This makes it difficult to attain a much higher printing speed as well as a strong printing force. In addition to the above, the permanent magnet 12 is in a constant magnetic circuit with the electromagnet of the core 13 and the coil 14 irrespective of the positions of the armature 15, reset or actuated. Accordingly, even if the electromagnet be utilized to overpower the permanent magnet, there should be required much electromagnetic force for repelling the armature to the actuated position against the attraction force resulting from the permanent magnet, which is disadvantageous in that the electromagnet requires much larger current so as to make slower the operation of the armature. Thus the prior art is found not to be satisfactory for effecting much higher armature operation as desired in a present-day dot printer.